The present invention relates to an injection unit for an injection-molding machine for processing thermoplastic material, the injection-molding machine having a screw which is guided in a cylinder and the axial movement of which can be initiated by a first motor and the rotary movement of which can be initiated by a second motor, an electrical direct drive being provided as the first motor.
An injection unit of this type is known from DE 43 44 335 A1. In this unit, polymer is forced out into a mold by a linear movement of a stationary screw. This operation takes place with a first direct drive. However, material preparation of the polymer has been initiated in each case beforehand by turning of the screw in an assigned cylinder with the aid of a second direct drive.
Direct drives have the advantage of having very good dynamics, one of the ways in which this is manifested being a very short time span to reach the injection speed. However, direct drives entail relatively high costs. This is because it is not only necessary for the motor to be adapted to the geometry of the machine but also to produce a very high torque at relatively low rotational speeds to provide the required injection speed with the desired injection pressure.
In the case of direct drives, both the injection motor and the metering motor have to supply a relatively great torque, which is derived, inter alia, from the injection pressure. The second motor, i.e. the metering motor, only moves, however, at rotational speeds which are significantly below the operationally required rotational speed of the injection motor. For example, one possible configuration is designed in such a way that the injection motor has to produce a torque of 1000 Nm at a maximum rotational speed of 1000 rpm, whereas the metering motor has to provide a torque of 1000 Nm for a rotational speed of below 300 rpm.
In commercially available machines, the two movements are transmitted by indirect drives, using belts or gear mechanisms. The adaptation of the motor torque and motor speed to the requirements of the injection unit then takes place by the respective transmission ratio. In this case, the dynamics for automatic controlling operations are less, however, than in the case of direct drives, but use of standard drives can be made possible by the design of the gear mechanisms.
The object of the invention is to provide an injection unit of the type described above in which optimum conditions with respect to performance and cost-effectiveness are achieved for the overall arrangement.
The inventors have found that the dynamic requirements in the case of injection units of the type described above are indeed very high for the actual injection operation but not so high for the metering operation, the stated object can be achieved by only the second motor being connected to the screw via a gear mechanism, in such a way that the rotational speed of the second motor can be reduced to a lower speed of the screw, adapted to the material-preparing process.
A first advantageous design of the invention is characterized in that the gear mechanism can be blocked during the operation of injecting the thermoplastic material. This prevents the metering motor from having to supply a torque during injection.
In a technically extremely simple form, this block may be provided as a back stop, as shown in the figure described below.
An extremely low-cost configuration is obtained by the fact that a belt-pulley gear is provided as the gear mechanism.
The fact that the first motor rotatably moves a spindle, which is connected to a screw, via a displaceably secured spindle nut allows a rotating motor to be used as a drive for producing the axial movement in this respect. Consequently, unlike in the case of using purely linear drives, it is possible to fall back on standard components.